Dial methods return interface. Added a mock object/interface/New.
addressed CR comments
comment typo
added MockClient.NextStat()
updated mock comment
simplified interface, added error return value on NextStat
Remove MakePrefix method
Bump version 2.0.0, update CHANGELOG
Merge pull request #7 from sendgrid/remove-makeprefix

Remove makeprefix
Update CHANGELOG
Merge remote-tracking branch 'matthewaltman/interface_and_mock' into release/v2.0.0

Conflicts:
	statsd_test.go
Merge pull request #6 from sendgrid/release/v2.0.0

Release/v2.0.0